At first sight, one would assume that the tag is for works deemed to be classics. For example the works of Jules Verne or H.G.Wells.
The tag has been applied to 11 questions, none of which are any more "classic" than any other question on this site.

Ten of the 11 also carry story-identification and, for these, the classic tag seems irrelevant.
The eleventh is about Soylent Green, the classic status of which is open to debate.

Should we clean up the tag? Create a defining wiki?
At present the tag appears to be confusing.

Edit: The tag has now been removed from all questions. Can it be deleted?

Purely by the name, I assumed that classic-scifi referred to pre-1923 stories, those whose copyright has expired in the United States (the home country of Stack Exchange). That would include most famous Wells stories but not Tolkien and not Soylent Green, which is based on Harry Harrison's Make Room! Make Room! (1966). This has precedent elsewhere; TV Tropes uses copyright expiry as a trigger for when a work is free to spoil.

But is there a reason why one would be an expert on SF works that, say, happen to be available on Project Gutenberg?